I found this info that might help. My friend took in some rescue horses and it took her a couple years before they filled out right. What does the vet say? Sounds like a lovely horse -- see if this info does anything to help you out.
Today, equine nutritionists have turned to fats as a far more efficient and advantageous caloric alternative. Fats offer more than twice the calories per gram than do grains. A pint of corn oil added to the daily ration will significantly boost the caloric intake (3840 calories per pint) without adding bulk. By contrast, one pound of typical sweet feed will supply 1100-1600 calories and one pound of corn approximately 1600 calories.
Fats offer other benefits as well. Containing neither the fiber nor the carbohydrates of the grains, fats produce little internal heat during digestion and are not generally associated with the unruly behavior sometimes linked to feeding high carbohydrate grains like corn. There is some concern, however, that fats comprising more than fourteen percent of the ration may inhibit proper utilization of the fat-soluble vitamins. Both vegetable and animal fat products are available in either liquid or dried forms, although vegetable fats are generally more appetizing to the equine palate. There are several newer commercial grain mixes with higher fat levels now being marketed specifically for hard keepers and horses in heavy work.
If you decide to add fat to your horse's diet, be sure to incorporate it gradually. I start my horses with 1/8 cup of oil twice a day mixed with their grain. After a week, I will increase it to 1/4 cup twice a day. In another week, I will increase it to 1/2 cup twice a day, if necessary. You can give up to a total of 2 cups a day, but if you start a horse at that level, you'll be sure to give him a major case of diarrhea. His system needs time to adjust to the fat. You will also need to keep a very close eye
on his weight if you start feeding fat. Horses can gain weight quickly on fat, and the risk of laminitis is high if weight is gained rapidly and in excess. Before you make any major dietary changes, though, you might want to get the vet out to take a look at this horse.